A number of electronic safety devices are known, which are adapted for use with protection barriers for industrial plants, and are designed to open an electric circuit in case of emergency or hazard for a user.
Particularly, such known devices comprise a first portion associated with a stationary part of the barrier and a second portion associated with a moving part of the barrier, e.g. a protection member, a door or the like, to monitor the opening and closing states thereof and stop the operation of the plant if the barrier is open.
Namely, the device comprises one or more switches, which are designed to change their own electrical state from closed to open or vice versa, in function of the distance between the stationary portion and the moving portion.
For instance, the switch may be electrically closed only if the two parts are at very little or no distance from each other.
A particularly common type of such safety devices uses a RFID transponder associated with the moving portion and is adapted to communicate with a transceiver device anchored to the stationary part and having a reading head which is adapted to receive an identification signal sent by the transponder and compare it with a code stored therein.
Thus, when both codes match, the device closes the circuit thereby allowing operation of the plant.
This configuration prevents any accidental or fraudulent actuation, because communication between the transceiver device and a well-defined transponder is needed for the circuit to be closed.
Nevertheless, these solutions also have drawbacks, as any interference might cause reading errors and resulting failures.
For overcome such drawbacks, forms of such safety devices have been developed, with control circuits designed to prevent any malfunctioning.
For instance, EP0968567 discloses an electronic safety device of the type as described hereinbefore, in which the microprocessor control circuits designed to check whether the identification code transmitted by the transponder matches the code stored in the reading head are provided in duplicate.
Particularly, each control circuit is designed to individually check the received signal and compare the received identification code with the stored code.
The actuation signal will be only generated if the received identification code matches the stored code for both control circuits.
While this device provides a higher safety degree, due to the redundancy of the control circuits, it still suffers from certain drawbacks, one of which is the apparent circuit complexity required by the duplication of all the control elements.
Furthermore, the distance between the transponder and the reading head is assessed by detecting the amplitude of the received return signal, which may easily change due to spurious interferences in the area around the device.
Furthermore, the control circuit directly operates on the waveform of the received signal, and this might introduce distortions that might significantly change the amplitude value, particularly in case of malfunctioning.
A further drawback of this solution is that the received signal is poorly immune to background noise and such signal can be only properly detected by providing the control circuit with highly effective and relatively expensive filters, which must be further centered in a particularly accurate manner around the signal transmission frequency, with poor adaptability of the filtering features to the various types of commercially available transponders.
Furthermore, the provision of a device for monitoring the amplitude of the received signals and the checking elements further reduces the possibility of using various types of RFID transponders, with different transmission frequencies.
Finally, the device for monitoring the level of the received signal is connected downstream from the antenna of the reading head and may be affected by the temperature of the outside environment in which the electronic safety device is installed.